"The Iranians have sought to mask their activities," says one official. Another official notes the US is convinced Iran is secretly trying to develop nuclear weapons in violation of the Nuclear Non-Proliferation Treaty. Adds he: "We think they are prepared to cheat on the CWC ... too."

Iran was one of 164 states that signed the CWC in 1991 and says it intends to join the 81 states that have now ratified it. Iran denies US charges that it seeks chemical, biological, and nuclear weapons in a bid to become the dominant power in the Persian Gulf region, where a 20,000-strong US military force safeguards the free flow of the world's largest petroleum reserves.

The possibility of Iran trying to maintain a secret program even while joining the CWC was among the arguments made by opponents, who fought unsuccessfully to block the treaty's ratification by the Senate last Thursday. They warned that even with the most rigorous verification and inspection procedures of any arms control accord ever concluded, the CWC won't prevent US foes from maintaining clandestine stockpiles.

Clinton administration officials make it clear that as a member of the executive committee of the new UN treaty enforcement organization, the US will press for vigorous probes into any suspected breaches by Iran. Says a senior official: "Our intention is to pursue violations with a vengeance."

The CWC bans development, possession, and use of chemical weapons. States that ratify it must declare to the UN Organization for the Prohibition of Chemical Weapons (OPCW) any chemical arms they have and agree to destroy them in a specified period. They must also disclose civilian plants that make chemicals for civilian and military use.

The OPCW will verify declarations of member states with routine inspections. Members who suspect violations can request more intrusive probes called "challenge inspections." These can be staged on short notice with the approval of one-fourth of the states on the OPCW executive committee.

But detecting clandestine chemical-weapons programs may still be hard. So-called "precursor" chemicals are often made in civilian plants and also used to make numerous consumer products. Weapons-production facilities can also be "embedded" in civilian plants. Such is the case with Russia.

It is also believed to be the case with Iran, which reportedly purchased nerve-gas technologies from China several years ago.

In February, acting CIA Director George Tenet told a Senate committee that Iran "has sought the capability to produce not only the chemical agents themselves, but also the precursor chemicals, making it less vulnerable to export controls."

Other officials, speaking on condition of anonymity, go further. They say Tehran has produced "sizable" quantities of chemical agents. They decline to provide further details. Says one: "The key question is whether they will declare and destroy their stockpiles as they are required to do under the CWC."

US officials say that after ratifying the CWC, Iran's declarations to the OPCW will be closely scrutinized. If discrepancies are found, they indicate the US will almost certainly press for challenge inspections.

"Iran has not ratified yet," says one official. "If they fail to ratify, that will be prima facia evidence consistent with our concern. If they ratify, that will provide a mechanism for going and finding some things out."

CWC violators are liable to a range of sanctions. They run from a ban on doing business in chemicals with treaty members to the imposition of full-scale sanctions.

Currently only the US and Russia acknowledge having the weapons. Both are bound to destroy theirs under a 1990 bilateral pact. The US believes as many as 25 nations have undeclared stores, including Iran, China, Syria, North Korea, and Libya. The latter three have declined to join the CWC.

If challenge inspections in Iran are approved, it could still be hard for the OPCW to ferret out chemical-weapons plants. But one sign, says the senior official, would be excessive amounts of safety and decontamination equipment in a civilian factory.

Iran would have the right to demand that US experts be excluded from OPCW inspection teams. In that event, the inspectors would be briefed as fully as possible on what to look for by US officials.

CHEMICAL WEAPONS CONVENTION DETAILS

The Convention, which goes into effect April 29, has been signed by 164 countries and ratified by 81, including the US. The pact will: